1. Field of the Invention
The present invention relates to a method and an apparatus for correcting the circumferential length of a metal ring that constitutes a metal belt or the like of a continuously variable transmission.
2. Description of the Related Art
There have been known conventional metal belts used for continuously variable transmissions that are composed of a stack of metal rings having slightly different circumferential lengths. The metal rings are fabricated by welding the opposite edges of a thin plate of an ultra high strength steel, such as maraging steel, into a cylindrical shape, subjecting the cylinder to a first solution treatment, cutting the cylinder into rings having a predetermined width, and then rolling the resulting rings. Then, the metal rings are subjected to a second solution treatment. However, the second solution treatment causes variations in circumferential length of the rings. Thus, to form a metal belt by stacking such metal rings, the circumferential lengths of the metal rings have to be corrected.
A conventionally known circumferential length correcting apparatus comprises a driving roller for rotational driving, a driven roller capable of being displaced to move away from the driving roller, control means for controlling the displacement of the driven roller, and a circumferential length correcting roller that is positioned between the driving roller and the driven roller to correct the circumferential length of a metal ring passed around the driving roller and the driven roller (International Publication No. 2002/038302, for example).
When the circumferential length correcting apparatus corrects the circumferential length of a metal ring, the metal ring is first passed around the driving roller, the driven roller and the circumferential length correcting roller, and the control means displaces the driven roller away from the driving roller to tighten the metal ring. Then, with the driving roller being rotationally driven, the displacement of the driven roller is measured while keeping the metal ring under tension, and the control means calculates the actual circumferential length of the ring from the center distance between the driving roller and the driven roller.
Then, from the difference between the calculated actual circumferential length and the desired circumferential length, the control means calculates a displacement of the circumferential length correcting roller required to correct the circumferential length of the ring to the desired circumferential length.
Then, the control means biases the circumferential length correcting roller upward. At this time, the driven roller moves toward the driving roller by the action of the upward biasing force applied to the circumferential length correcting roller, and then the driving roller and the driven roller are held at a predetermined distance. In this state, if the circumferential length correcting roller is biased upward, the metal ring is plastically deformed, and the circumferential length correcting roller is gradually displaced upward as the plastic deformation proceeds.
Then, the control means corrects the circumferential length of the ring by displacing upward the circumferential length correcting roller until the displacement of the circumferential length correcting roller becomes equal to the calculated displacement described above.
In addition, the circumference of the circumferential length correcting roller has an arc shape with the center protruding outward in lateral cross section. Therefore, when correcting the circumferential length of the ring, an arc shape conforming to the shape of the circumference of the circumferential length correcting roller is imparted to the metal ring at the same time. When forming a metal belt by stacking such metal rings thus arc-shaped, the stack state of the rings can be readily maintained because the imparted arc-shaped surfaces are engaged with each other.
However, in the case where the arc shape is imparted to the metal ring while correcting the circumferential length of the metal ring, there are disadvantages that adjusting the circumferential length to an appropriate value may result in inappropriate imparting of the arc shape to the metal ring and that appropriately imparting the arc shape to the metal ring may result in a shift of the circumferential length from the appropriate value.